Influenza is a major health threat throughout the world, causing hundreds of thousands of deaths during inter-pandemic years and millions of deaths during pandemic years. Despite this, little is known about the epidemiologic features, seasonality, and transmission of influenza in tropical developing countries. To characterize influenza in a tropical developing country, we propose to conduct an International Collaboration of Infectious Disease Research (ICIDR) program consisting of two related studies in Managua, Nicaragua: a prospective cohort study of the epidemiologic features of influenza in children and a study of the household transmission of influenza. These studies will benefit from 21 years of collaboration between the study investigators at UC Berkeley and the Nicaraguan Ministry of Health and from the scientific capacity building in Managua that has resulted from numerous studies on dengue, and, more recently, influenza, which will be continued through the ICIDR program. Specifically, this program will extend a pediatric influenza cohort study that has already been ongoing for two years, with high-quality laboratory, epidemiological, clinical, and operational infrastructure, extensive community buy-in, and enthusiastic support from the Nicaraguan Ministry of Health. This ICIDR will be composed of two interrelated projects: Project 1 focuses on the epidemiology and phylogenetic analysis of pediatric influenza in the cohort, and Project 2 consists of a longitudinal household transmission study of influenza. Within Project 1, the burden and seasonality of influenza in the cohort will be examined, risk factors for influenza virus infection and influenza severity will be characterized, and the evolutionary dynamics of influenza A virus will be evaluated over time and space, including an investigation of intra-host diversity, using viruses obtained from both the cohort and the household transmission studies. In Project 2, households with an index case of influenza will be studied to investigate parameters of influenza transmission in a developing country setting, to determine the serial interval, and to examine risk factors that affect influenza transmission and disease severity. This ICIDR proposal is well-positioned to have substantial impact because of the timeliness of the emerging pandemic of Influenza A H1N1 of swine origin and the lack of knowledge of the epidemiology, and transmission of influenza in tropical countries, which the proposed studies will address. RELEVANCE : Influenza is a major public health problem worldwide;here we examine the epidemiologic features and transmission of influenza in a pediatric cohort in Nicaragua. Despite the importance of the tropics in global influenza circulation, few data exist on influenza in tropical developing countries. This is a unique opportunity to answer critical questions about a disease of global importance at a particularly important time, coinciding with the emergence of Influenza A H1N1 of swine origin.